Automated fiber placement machines are widely used to manufacture parts, components and structures from composite material. The materials used in automated fiber placement are typically composed of longitudinal fibers and resin consolidated into tapes, or thin strips, commonly known as “tows.” Individual tapes or tows are manipulated by the fiber placement machine to form a band of material that is deposited onto a tool. Parts are built up layer-by-layer, with tapes or tows of composite material. The angle at which each layer or “ply” is laid onto the tool is precisely determined by the fiber placement machine.
Automated fiber placement enables the construction of complex composite structures having steered or curvilinear fiber paths. This method of producing composite structures is more cost effective than manual methods. It provides an improved structural efficiency due to its ability to orient the fibers along local internal load paths, which potentially results in lighter structures and lower costs than in structures made by other production methods.
The tows of material are actually laid onto the surface of a tool or a composite part being formed by a fiber placement head. The fiber placement head includes a compaction roller for pressing the tows against the surface of the tool or the preceding layers of material forming the composite part. In one way of using compaction rollers, disclosed in U.S. Pat. No. 4,867,834, a plurality of outer wafers or rollers with a single diameter are used with sprung arms to apply compaction force to an article that is being filament wound. This technique does not help for parts lacking radial symmetry such as prismatic parts. U.S. Pat. No. 5,110,395 also discloses a fiber placement head and a tool for compacting the composite tows laid down. This tool include a compaction shoe with a single-diameter outer surface that is urged against the part being fabricated by several pistons and segmented cylinders within the compaction shoe. While this method is useful for radially-symmetric composite parts, such as oxygen bottles or motor housings, it would be difficult to apply even pressure to parts lacking this symmetry.
In one improvement on these methods, U.S. Pat. No. 4,869,774 discloses a compaction tool made from a plurality of discs or wafers, also with a single outer compaction surface. An inner bladder extends through each of the discs. One of the discs is fixed radially to the compaction tool, while the others are allowed to move independently up or down with respect to the compaction surface, in response to the contours of the part being fabricated and the pressure in the bladder. While this is an improvement, the single compaction surface does not allow the segments to move sufficiently freely to insure compaction in areas where there is a significant change in the diameter or dimension of the part being fabricated.
U.S. Pat. No. 5,454,897 also discloses externally-facing, segmented pressure members for compacting a part being fabricated. The presser tool includes a central segment that is fixed, with additional segments on each side of the center and a pressure bladder on each side. The segments have an outer low friction surface, an elastomeric compression layer, and an outer roller bearing that allows each segment to rotate independently. The central segment or segments are fixed, and are unable to move in or out with respect to the other segments. Thus, tools made according to this patent will not have freedom of movement at least in the central portion of the tool, and the corresponding portion of the part being compacted may not receive sufficient pressure to compact the material.
U.S. Pat. No. 6,390,169 also discloses a compaction apparatus with two internal pressurizing bladders and a plurality of external segments, each with an outer roller bearing and compaction surface. A pivoting shaft extends through the segments, which have cutouts that allow the segments to pivot on the shaft while allowing the individual segments to move in and out, except for the fixed central segment. The fixed segment limits the ability of the tool to conform to the part surface and compact the composite material.
It is desirable to provide an improved compaction roller for the fiber placement head of an automated fiber placement machine having a higher degree of compliance, with uniformly applied compaction force, than has heretofore been achievable with prior compaction roller designs. It is further desirable to provide an improved compaction roller in which the compaction force can be selectively modified over the length of the roller. It is also desirable to provide a compaction roller of simple straight-forward design and operation.